1. Field of the Invention
The present invention relates to golf balls. Particularly to golf balls having five layers including a dual core, a dual mantle and a thermoplastic polyurethane cover.
2. Description of the Related Art
Sullivan et al., U.S. Pat. No. 4,911,451, for a Golf Ball Cover Of Neutralized Poly(ethylene-acrylic acid) Copolymer, discloses in Table One a golf ball having a compression of below 50 and a cover composed of ionomers having various Shore D hardness values ranging from 50 to 61.
Sullivan, U.S. Pat. No. 4,986,545, for a Golf Ball discloses a golf ball having a Rhiele compression below 50 and a cover having Shore C values as low as 82.
Egashira et al., U.S. Pat. No. 5,252,652, for a Solid Golf Ball, discloses the use of a zinc pentachlorothiophenol in a core of a golf ball.
Pasqua, U.S. Pat. No. 5,721,304, for a Golf Ball Composition, discloses a golf ball with a core having a low compression and the core comprising calcium oxide.
Sullivan, et al., U.S. Pat. No. 5,588,924, for a Golf Ball discloses a golf ball having a PGA compression below 70 and a COR ranging from 0.780 to 0.825.
Sullivan et al., U.S. Pat. No. 6,142,886, for a Golf Ball And Method Of Manufacture discloses a golf ball having a PGA compression below 70, a cover Shore D hardness of 57, and a COR as high as 0.794.
Tzivanis et al., U.S. Pat. No. 6,520,870, for a Golf Ball, discloses a golf ball having a core compression less than 50, a cover Shore D hardness of 55 or less, and a COR greater than 0.80.
Core durability is a limiting factor as to how large of a compression differential you can build into the construction. In a conventional 4-piece construction a dual core with a high compression differential can have a tendency to fail (ie. break) if it experiences high impact forces. The impact forces cause high stress forces in the mantle layer which are focused around the stress concentrations introduced in the injection molding process, most notably around the pins and gates. When the stress increases beyond the tensile strength of the material the protective mantle layer fails and as a result the core sees the majority of the force and cracks.
A standard test that is used to evaluate this is the PTM cannon which fires ball into a rigid, steel plate at 200 fps. The threshold for acceptable durability is a Mean Time to Failure (MTTF)≧30 blows with no breaks below 10 blows. This test is conducted with 24 balls and plotted using a Weibull plot to establish these values.